1. Field of the Invention
The present invention relates to a database access system, and particularly relates to a database system where a hierarchical network database system is treated as a relational database system in accessing operations.
2. Description of the Prior Art
A database system of a hierarchical network type is capable of supporting various data structures. It is suitable for routine work and provides high processing efficiency. When developing an application program, however, the development engineer must be quite familiar with the data structure and the user interface tends to be complicated. On the other hand, a relational database system has a simple data structure. Since it expresses the data in table format, it provides a simpler user interface. It is suitable for non-routine work, but gives only low processing efficiency.
For the above reason, some systems combining the practical feature of a hierarchical network database system and the user-friendly interface of a relational database system, i.e. database systems where a hierarchical network database system is accessible with the DML (Data Manipulation Language), which is used for a relational database system, have been proposed.
Such conventional systems can be classified into three types. The first type system is provided with a pre-compiler, which allows input of a relational type DML queny and generates a DML source program of the hierarchical network type. With the second type, the relational type DML query is input to the system, which directly generates a DML object program of the hierarchical network type. For the third system, an index is given in advance to each physical record in the database files. The system is provided with a means to access the database using the indices, which is activated by input of relational type DML queny and generates an object.
With the first system described above, a DML source program of the hierarchical network type is generated from a relational type DML query. For this, it is necessary to generate another source program for condition judgment based on the execution results of the hierarchical network type DML. This means that complicated conditions for judgment, i.e. a complicated data processing request, require a complicated source program in a larger scale to be generated, which results in poor processing efficiency.
In the second system, a DML object program of the hierarchical network type is directly generated from relational type DML. However, as in the case of the first type system, the generated object program becomes larger and complicated as the data processing request becomes complicated, resulting in poor processing efficiency.
With the third system, since an index is given to each physical record in the database files, an area for index provision must be ensured separately in the database. This results in a larger capacity of the database. Further, the record access mechanism of the hierarchical network type database cannot be utilized without indices. Still another drawback of the third type system is that the embodiment is limited to a database having 1 schema, 1 range or 1 record type.